Enhanced photoinduced mass migration in supramolecular azopolymers by H-bond driven positional constraint

Article abstract of DOI:10.1039/d1tc02266k

Here we investigated the role of hydrogen bonding in the design of supramolecular azopolymers with a highly directional and constrained azobenzene-chain interaction involving the aromatic ring of the photoactive molecule, by exploiting the 2-aminopyrimidi

Full text of DOI:10.1039/d1tc02266k

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Materials Chemistry C
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Enhanced photoinduced mass migration
in supramolecular azopolymers by H-bond driven
Cite this: DOI: 10.1039/d1tc02266k
positional constraint†
ab
bc
b
Fabio Borbone,
Filippo Pota,
*
Stefano Luigi Oscurato,
Salvatore Del Sorbo,
a
c
c
Marcella Salvatore,
Francesco Reda,
bc
a
b
Pasqualino Maddalena,
Roberto Centore
and Antonio Ambrosio
*
Here we investigated the role of hydrogen bonding in the design of supramolecular azopolymers with a
highly directional and constrained azobenzene–chain interaction involving the aromatic ring of the
photoactive molecule, by exploiting the 2-aminopyrimidine/carboxylic acid supramolecular synthon as
the tool for molecular recognition. We have shown that this approach is advantageous for producing
affordable and versatile photopatternable azomaterials by complexation with polyacrylic acid (PAA).
Molecular model complexes were successfully prepared and characterized by X-ray diffraction analysis
and FTIR spectroscopy to reveal the multiple, non-ionic interaction occurring between the azobenzene
units and the polymer chains. Surface photopatterning of thin films, driven by the typical mass migration
phenomenon occurring in azopolymers, resulted strongly enhanced with increasing azobenzene
content until equimolar composition. Results show that polymers with synthon-based azobenzenes
markedly outperform single H-bonded systems bearing azomolecules with similar structure and
electronic properties. We finally demonstrated that the azobenzene units can be easily extracted from a
photopatterned film by a simple solvent rinse and without any chemical pre-treatment, leaving the
periodicity of the inscribed surface relief gratings unaltered. This result was enabled by the orthogonal
solubility of the components in the supramolecular system.
Received 17th May 2021,
Accepted 15th July 2021
DOI: 10.1039/d1tc02266k
rsc.li/materials-c
to drive the orientation of mesophases in liquid crystals based
Introduction
2
–5
materials or in optical devices for reversible holographic data
6
–9
Materials functionalized with azobenzene or its derivatives storage.
Macroscopic mass migration occurring on the sur-
have been the object of extended research for decades because face of azopolymers thin films is recognized as another impor-
1
of their ability to respond to light. The absorption of radiation tant phenomenon resulting from trans–cis photoisomerization
1
0
with proper wavelength gives rise to photoisomerization of the in azomolecules. Irradiation of the surface with an interfer-
azomolecules between trans and cis isomers showing a strong ence light pattern of polarized laser beams can induce highly
conformational difference. The possibility of sustaining the directional polarization dependent mass transport, with for-
continuous trans–cis–trans cycle through irradiation is at the mation of topographic modulations known as surface relief
origin of the reorientation of the azomolecules under illumina- gratings (SRG), which replicate the illuminating intensity
1
1
tion with a linearly polarized laser beam. The resulting induced pattern and can be erased by irradiation with circularly
1
2
anisotropy and birefringence can be advantageously exploited polarized light or by heating. The result is a single-step all-
optical reversible process to control surface topography which
can be exploited in numerous technological applications, from
a
Department of Chemical Sciences, University of Napoli Federico II,
1
3,14
photonics and optoelectronics such as DFB lasers
and
tem-
Complesso Universitario di Monte Sant’Angelo, Via Cintia, Naples 80126, Italy.
E-mail: fabio.borbone@unina.it
1
5
16,17
organic solar cells to complex textured surfaces,
b
c
18
19
CNST@POLIMI – Fondazione Istituto Italiano di Tecnologia, Via Pascoli 70,
Milan 20133, Italy. E-mail: antonio.ambrosio@iit.it
plates for lithography or tuning of wettability. In a poly-
meric material, mass migration and SRG formation efficiently
occur when azochromophores are linked to the polymer chain,
thus mainly covalently bonded side-chain and main-chain
polymer systems were initially investigated. However, in the
last decades the supramolecular approach has emerged as a
Department of Physics E. Pancini, University of Napoli Federico II,
Complesso Universitario di Monte Sant’Angelo, Via Cintia, Naples 80126, Italy
Electronic supplementary information (ESI) available. CCDC 2073781–2073786.
†
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/d1tc02266k
1
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powerful tool in the design of azomaterials, because of the high
flexibility offered by simple synthetic procedures, mainly based
on mixing components in solution, the possibility to investigate
a wide range of noncovalent interactions of variable strength
and the effortless optimization of composition. Indeed, differ-
ent types of supramolecular chromophore–chain interactions
have been successfully exploited to produce efficient SRG
formation, from non-directional and strong ionic bonds to
2
0–27
hydrogen and halogen bonding.
based on halogen-bonded azochromophores grafted onto poly-
-vinylpyridine have revealed interesting performances in
Supramolecular polymers
4
terms of SRG inscription efficiency and in some cases out-
performed hydrogen-bonded systems based on the same photo-
passive polymer, although they suffered from phase separation
2
8
at high azochromophore content. The reason of the higher
efficiency has been ascribed to the high directionality of the
halogen bond, in which the s-hole on the aromatic halogen
involved in the electrostatic interaction is confined on the
elongation of the R–X covalent bond axis, resulting in a more
rigid chromophore–chain connection, improved ‘‘dragging’’ of
the chains by the photoactive azomolecules during isomeriza-
tions and hence better overall mass migration compared to
conventional H bonded systems. Moreover, given the same direc-
Fig. 1 Chemical structure of azochromophores (a) and grafted polymers
(
b and c).
29
tionality, efficiency increased with halogen bond strength.
Herein we propose a model of a supramolecular azopolymer in
which the degrees of freedom in the relative motion of azoben- the corresponding AzoꢀHBF
4
product, then deprotonated and
zene unit and polymer chain are further reduced through the crystallized by treatment with triethylamine in ethanol/water.
direct involvement of the azobenzene aromatic ring in an addi- These compounds were suitable for grafting onto polyacrylic
tional weak bond. We investigated the role of hydrogen bonding acid (PAA) through complexation based on the supramolecular
in increasing strength and directionality of the chromophore– heterosynthon showed in Fig. 1b. It has been recently demon-
chain interaction and the effect on the efficiency of SRG for- strated that this polymer is able to produce very stable amor-
mation, by exploiting the action of cooperative multiple hydrogen phous solid dispersions of drugs containing a 2-aminopyridine
bonds. In this regard, we used the concept of the supramolecular based pharmacophore. A second type of PAA based polymer
synthon as a design element to provide a strong molecular was also prepared by using compound 3, with the aim to
recognition system for a stable and directional grafting. The compare the mass migration performances of synthon-based
double hydrogen bond occurring between a carboxylic acid group polymers with those of a similar system characterized by a
and a 2-aminopyridine type moiety is recognized as a robust single chromophore–chain hydrogen bond (Fig. 1c). To inves-
supramolecular heterosynthon in crystal engineering and proved tigate at a molecular level the guest–host interactions occurring
as a useful tool to produce stable amorphous solid disper- in the PAA-1/2 azopolymers we first studied the crystal structure
30,31
sions.
In this work, we designed new supramolecular of the azopyrimidine derivatives and that of model compounds
azopolymers by grafting 2-aminopyrimidine functionalized azo- of the azopyrimidine–polymer system by single crystal X-ray
chromophores onto polyacrylic acid (PAA) and investigated the diffraction. Compound 1 was crystallized in two polymorphs,
32
efficiency of SRG formation. A comparative study of SRG inscrip- named 1-I and 1-II, while 2 as one phase. The crystal packing of
tion performances with a single H bonded system revealed a very both the polymorphs of 1 and of compound 2 is driven by the
strong influence of the supramolecular synthon on the efficiency common motif of double hydrogen bonds between the NH
2
of mass migration.
donor and two OCH
aromatic nitrogen of the pyrimidine ring as possible acceptor
Fig. S1–S3, ESI†). This pattern of interactions changes when
carboxylic monomers were used to induce molecular recogni-
tion. Compound was successfully co-crystallized with
methacrylic acid (MA-1) to obtain a monomeric model com-
3
acceptors, with no involvement of the
(
Results and discussion
Synthesis and structural characterization
1
The azochromophores 1 and 2 reported in Fig. 1 were synthe- pound of the PAA-1 polymer. A dimeric model compound of the
sized in high yields and purity through a simplified single step polymer was also obtained by co-crystallization with bifunc-
procedure (see Schemes S1 and S2, ESI†). In brief, diazonium tional adipic acid (AA-1). Single crystals were grown by evapora-
salts (isolated as tetrafluoroborates) were directly coupled onto tion from chloroform/heptane in both cases with high yields.
2
-amino-4,6-dimethoxypyrimidine in dichloroethane, to give X-Ray diffraction analysis revealed a 1 : 1 ratio for the complex
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MA-1. The azomolecule is in almost planar conformation, and
2
2
there is no disorder in the azo group. A very stable R
(8) cyclic
structure with eight atoms and two hydrogen bonds is formed.
This structure, in principle, can be realized in three ways: between
two carboxylic acid groups (AA homosynthon), between two
aminopyrimidine groups (PP homosynthon) and between an acid
and an aminopyrimidine group (AP heterosynthon).
The azomolecule in MA-1 is involved in two double hydrogen
bonding patterns (Fig. 2), an AP heterosynthon and a PP
2
2
homosynthon, and two R
(8) ring patterns are formed. In the
PP homosynthon, the N–Hꢀ ꢀ ꢀN hydrogen bond shows Nꢀ ꢀ ꢀN
and Hꢀ ꢀ ꢀN distances of 3.172(2) Å and 2.32(2) Å respectively,
while the bond angle is 168(2)1. In the AP heterosynthon, for
the N–Hꢀ ꢀ ꢀO bond, Nꢀ ꢀ ꢀO and Hꢀ ꢀ ꢀO distances are 2.836(2) Å
and 1.91(2) Å respectively, while the angle is 175(2)1; for the
OHꢀ ꢀ ꢀN bond, Oꢀ ꢀ ꢀN and Hꢀ ꢀ ꢀN distances are 3.051(2) Å and
2
(
.19(2) Å respectively, with an angle of 175(2)1. The resulting
MA-1) dimers are laterally packed to form infinite layers.
These layers are parallel to the lattice planes of Miller indices
%21 and stacked with a spacing of 3.28 Å. Crystals of the AA-1
complex revealed a melting peak at 138 1C in the DSC diagram,
2
1
Fig. 3 Partial packing of AA-1. (a) Face view with H bond pattern in
which is lower than those of the two components (152 1C for orange; (b) edge view.
both adipic acid and 1). The complex AA-1 crystallized in a 1 : 2
ratio, with each adipic acid molecule involved in two AP
heterosynthons with two azomolecules (Fig. 3). Each of these the AP heterosynthon, for the N–Hꢀ ꢀ ꢀO bond, Nꢀ ꢀ ꢀO and Hꢀ ꢀ ꢀO
two azomolecules are linked through a PP homosynthon to the distances are 2.895(2) Å and 2.04(3) Å respectively, while the
azomolecule of another AA-1 complex, giving rise to a 1-D angle is 174(2)1; for the OHꢀ ꢀ ꢀN bond, Oꢀ ꢀ ꢀN and Hꢀ ꢀ ꢀN
polymeric chain through alternated PP and AP synthons. These distances are 2.974(2) Å and 2.18(2) Å respectively, with an
chains are packed to form layers as in MA-1, which are stacked angle of 170(2)1.
with a spacing of 3.38 Å. In the PP homosynthon, the N–Hꢀ ꢀ ꢀN
The corresponding bond distances in the two complexes are
hydrogen bond shows Nꢀ ꢀ ꢀN and Hꢀ ꢀ ꢀN distances of 3.207(2) Å similar and show that the hydrogen bonds in the heterosyn-
and 2.41(2) Å respectively, while the bond angle is 167(2)1. In thons are shorter, as expected due to the higher acid–base
character of this interaction. However, the lengths of CQO and
C–O bonds in the carboxylic group (1.198 Å and 1.334 Å
respectively for AA-1, 1.209 Å and 1.333 Å respectively for
MA-1) are in the normal range for fully double and single bond
respectively and suggest that no proton transfer occurs from
hydroxyl to the nitrogen of pyrimidine ring. To support this
observation, we performed a statistical analysis of all the
compounds containing the same 2-aminopyrimidine/carboxylic
acid heterosynthon reported in the CSD database. The mean
values of CQO and C–O bond lengths of the 143 hits reported
are 1.2127 Å (s = 0.0172) and 1.3084 Å (s = 0.0136) respectively
and confirm that formation of carboxylate ion is not expected
in this type of heterosynthon. The X-ray analysis confirmed the
robustness and non-ionic character of the supramolecular
synthon and supported the assignment and comparison of
FT-IR signals (vide infra).
Spectroscopic characterization
The azochromophores 1 and 2 were used to prepare the PAA
based supramolecular polymers PAA-1 and PAA-2 with x = 0.5,
x
x
0.75 and 1.0 dye/polymer molar ratio, by dissolving the proper
amount of azochromophore and PAA (M 1800) in dimethyl-
w
formamide (DMF), in order to have a 1 : 3 azopolymer/solvent
weight ratio. The solutions were spin-coated on glass slides or
Fig. 2 Partial packing of MA-1. (a) Face view with H bond pattern in
orange; (b) edge view.
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KBr pellets for characterization. All the solutions gave rise to involved in the AP heterosynthon observed in the crystal
perfectly transparent and amorphous films, without any evi- structure. Evidence of the second hydrogen bond involving
dence of phase separation or dye aggregation, even for the the amino group can be gathered from the analysis of the
highest chromophore content, corresponding to 79.1 wt% and characteristic NH stretching and bending frequencies. In
ꢁ
1
8
0.1 wt% respectively for PAA-11.0 and PAA-21.0. The UV-vis the spectra of pure 1, the band at 1622 cm corresponds to
absorption spectra of polymers in DMF solution and thin films the typical scissoring of the NH group, as known for amines
show maxima at 359 nm and 364 nm for respectively PAA-1 and reported for 2-amino-4,6-dimethoxypyrimidine.
and PAA-2 , corresponding to the p - p* transition, with a bending mode is dependent on the hydrogen bonding and
shoulder at longer wavelengths corresponding to the n - p* occurs at higher vibrational energy with increasing strength
2
3
5
x
This
x
3
6
transition (Fig. S4, S5 and Table S3, ESI†). The spectra of thin and number of hydrogen bonds. In AA-1, this band is con-
ꢁ1
films are similar to those in solution and exhibit an expected siderably shifted to higher frequency (1667 cm ) because the
slight broadening, indicating that no aggregation occurs in the amino group of the azochromophore is involved in two hydro-
films even at the highest concentration. The relative slight gen bonds of the PP and AP type synthons, as shown by the
increase of the visible band with decreasing azobenzene X-ray analysis. In the spectra of all PAA-1
x x
and PAA-2 polymers
ꢁ1
concentration observed in both polymer series can be attrib- (Fig. S9 and S10, ESI†) this signal is split at 1665 cm and
ꢁ
1
uted to the absorption by the hydrazone tautomer produced 1640 cm , suggesting that part of the amino groups in the
after the partial protonation of the azo groups by the increasing polymer matrix can participate in second hydrogen bond with
3
3,34
free carboxylic acid groups.
The same effect was observed in surrounding acceptors. The second evidence of hydrogen bond-
the UV-vis spectra of 1 and 2 in ethyl acetate added with acetic ing is given by the analysis of vibrational frequencies of the NH
acid (Fig. S6 and S7, ESI†), while in acetic acid the chromo- stretching. In the spectrum of 1, asymmetric and symmetric
ꢁ
1
ꢁ1
phores resulted completely protonated. The refractive index stretching are visible at 3461 cm and 3354 cm respectively,
ꢁ
1
dispersion of polymers was evaluated by ellipsometry (Fig. S8, while the weak band at 3222 cm typically results from the
ESI†). Values of n at the relevant wavelength of 633 nm ranged Fermi resonance coupling between an overtone of the NH
2
ꢁ1
from 1.638 of PAA-31.0 to 1.706 of PAA-21.0 (Table S3, ESI†). In bending/scissoring at 1600 cm
and the symmetric N–H
order to investigate the supramolecular interaction in the stretching mode. When the amino group is involved in hydro-
synthesized azopolymers, a comparative FT-IR analysis was gen bonds, the NH stretching shifts to lower frequencies and its
performed on azochromophores, model compounds and poly- oscillator strength increases significantly. This effect and the
mers. In the IR spectrum of pure PAA (Fig. S9, ESI†), absorption reduced energy gap with the overtone transition leads to a
ꢁ
1
at 1706 cm
is attributed to the carbonyl stretching of strong enhancement of the Fermi resonance coupling and to a
3
1
ꢁ1
carboxylic acid dimer in the polymer matrix. This signal is dramatic increase of the 3200 cm band intensity, dependent
shifted to 1725 cm in the grafted azomaterials (Fig. 4 and Fig. on the strength of the hydrogen bonds.
ꢁ1
37,38
This coupling is
S9, S10, ESI†), indicating that the carboxylic acid dimer group is clearly visible in the spectrum of AA-1, where the intensity of
ꢁ
1
ꢁ1
no longer present. The same band is observed at 1727 cm in the band at 3226 cm exceeds that of the double band at
ꢁ1
the spectrum of the AA-1 complex (Fig. 4). Therefore, this band 3385 cm . A very similar coupling and band pattern is
has to be assigned to the carbonyl of the carboxylic group observed for polymers (Fig. 4 and Fig. S9, S10, ESI†) and
confirms that the double hydrogen bond of the AP heterosyn-
thon effectively takes place within the grafted polymers. The
formation of the hydrogen bond between PAA and 3 in PAA-31.0
was verified by following the shift of the stretching of carbonyl
ꢁ
1
ꢁ1
from 1706 cm to 1716 cm and that of the free pyridyl ring
ꢁ1
(
1020 cm ) to higher frequency, with the appearing of a
second band at 1040 cm (Fig. S11, ESI†).
ꢁ
1
39
Efficiency of SRG formation
The efficiency of mass migration was investigated by perform-
ing SRG inscription experiments on thin films of the prepared
polymers. Structuring was carried out by irradiating the sam-
ples with an interference pattern obtained with p-polarized
beams, producing regular and high quality SRGs for all poly-
mers. A laser wavelength of 457 nm was used to work in a range
of relatively low chromophore absorption, with the aim to
compare the performances of synthon-based systems with
those of the conventional hydrogen bonded PAA-31.0 in a
regime not affected by thermal effects. Fig. 5a shows the growth
of the 1st order diffraction efficiency (DE) produced by the
forming grating as a function of time. The efficiency of mass
Fig. 4 FT-IR spectra of PAA-11.0 (green line), AA-1 (orange line) and
(blue line).
1
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series, reaching maximum average values of 420 and 355 nm
for PAA-11.0 and PAA-21.0 respectively. Table S3 (ESI†) reports
the corresponding values of DE.
We further compared the SRG formation performances of
PAA-21.0 and PAA-31.0 at 457 nm. Compounds 2 and 3 show a
similar structure and bear the same methoxy group on the
unbound ring of the azobenzene structure. Moreover, the same
position of the hydrogen bond accepting nitrogen on the
pyridyl and pyrimidyl rings is expected to produce the same
positioning of the azo unit respect to the polymer chain. The
comparison shows that PAA-21.0 is considerably more efficient
than PAA-31.0 (maximum modulation depth of 40 nm, Fig. 6),
although their absorption spectra do not differ significantly
(lmax of 364 nm and 343 nm for PAA-21.0 and PAA-31.0 respec-
tively, Fig. S14, ESI†). Therefore, we evaluated the lifetime of the
cis isomer for all the PAA-i1.0 polymers according to a literature
Fig. 5 (a) Diffraction efficiency of PAA-1
film thickness: 880–950 nm; irradiance: 175 mW cm ); (b) diffraction
efficiency of PAA-21.0 and PAA-31.0
d) maximum of diffraction efficiency for PAA-1
of composition.
x x
and PAA-2 (x = 0.5, 0.75, 1.0;
ꢁ2
4
0
;
(c) SRG inscription rate and procedure (Fig. S15, ESI†). The cis–trans thermal isomeriza-
ꢁ
1
(
x
and PAA-2
x
as a function tion rate of PAA-31.0 (1.40 ꢂ 0.03 s ) resulted more than three
ꢁ1
times higher than that of PAA-1 and PAA-21.0 (0.42 ꢂ 0.03 s
1
.0
ꢁ1
and 0.38 ꢂ 0.03 s
respectively), indicating that thermal
migration can be related to the rate of SRG formation, calcu- isomerization of the cis isomer is not a limiting factor for the
lated as the slope of the straight line obtained by fitting the performance of PAA-31.0. Conversely, the dramatic efficiency
data in the range of linear growth, corresponding approxi- difference suggests that the presence of a second hydrogen
mately to the section of curves between 10% and 30% of the bond involving a further position on the aromatic ring of the
DE maximum value attained at the end of the experiment. azobenzene entails, for the same polymer, a considerable
These data are reported in Fig. 5c and d for all the polymers as a increase in the ability of the photoactive units to drag the
function of the azobenzene content. The results show that the polymer chains during the structuring of the material. This
SRG inscription rate linearly increases with the concentration effect can be reasonably attributed to the nature of the used
of the azochromophore in the polymer and a regular growth supramolecular heterosynthon, which is at the same time a
trend is also observed for the maximum DE value, for both PAA- weak and reversible but highly directional and constrained
1
and PAA-2. Interestingly, for each composition PAA-1 notice- interaction, resulting in an improved conversion of energy into
ably outperforms PAA-2 (by 41%, 34% and 18% for x = 1.0, 0.75 effective mass transport.
and 0.50 respectively) although the only structural difference
between azochromophores 1 and 2 is the oxygen on the
Selective removal of azobenzene
terminal methoxy group of 2. This electron donor can act as We also investigated the possibility to extract the azobenzene
an additional hydrogen bond acceptor within the polymer units from the polymer after SRG inscription. This operation is
matrix and contribute to reduce the effectiveness of cis–trans highly desirable in azomaterials for technological purposes, as the
isomerizations on molecular and chain motions, thus affecting removal of the azo units expands the possibility of use in optical
overall mass migration. This different performance is also devices to the whole range of wavelengths where absorption by
41
evident by comparing the AFM cross-sectional profiles of SRGs chromophores would occur. Seki and coworkers proposed an H
inscribed on PAA-1 and PAA-2 in the same time lapse (Fig. 6, see bonding based supramolecular system in which the selective
also Fig. S12 and S13, ESI† for x = 0.75 and x = 0.50). The extraction of azobenzene units was successfully performed on a
modulation depth increased with the azo content for both patterned sample after crosslinking of the polymer matrix by
exposure to reactive vapors and subsequent rinsing with solvent.
Here we demonstrate that this operation could be successfully
performed on polymers with x = 0.50 without any chemical
reaction. Fig. 7 shows AFM topography and UV-vis spectra of a
patterned film of PAA-10.50 before and after rinsing the sample in
dichloromethane for 1 min (see Fig. S16 for PAA-20.50, ESI†). The
film appeared transparent after treatment and the absorption
band of the azochromophore disappears in the spectrum
(Fig. 7d). However, the profile of inscribed SRG is preserved,
although with some loss of quality, accompanied by an expected
volume shrinkage, with a visible reduction of the feature heights
from about 110 nm to 60 nm (Fig. 7c). This possibility is enabled
by the quite different solubility properties of PAA and 1. While 1 is
Fig. 6 AFM cross-sectional profiles of SRGs inscribed on PAA-11.0
orange line), PAA-21.0 (blue line) and PAA-31.0 (black line) in the same
time lapse.
(
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1-MA and 1-AA were grown by evaporation from chloroform/
heptane. One selected crystal of each compound was mounted
in flowing N2 at 173 K on a Bruker-Nonius Kappa CCD
diffractometer equipped with Oxford Cryostream apparatus
(graphite monochromated MoKa radiation, l = 0.71073 Å,
CCD rotation images, thick slices, j and o scans to fill asym-
metric unit). Reduction of data and semiempirical absorption
correction were done using SADABS program. The structures
4
2
were solved by direct methods (SIR97 program) and refined
2
by the full-matrix least-squares method on F using SHELXL-
4
3
44
2
016 program with the aid of the program Olex2. H atoms
bonded to C were generated stereochemically and refined by
the riding model, those bonded to O and N were found in
difference Fourier maps and their coordinates were refined. For
all H atoms, Uiso(H) equal to 1.2 Ueq or 1.5 Ueq (Cmethyl) of the
carrier atom was used.
Fig. 7 Selective extraction of 1 from SRG of PAA-10.50. Topographical
AFM image of SRG before (a) and after (b) rinse with solvent; cross-
sectional profiles (c) and UV-vis spectra of film (d) before (blue line) and
after (orange line) rinse with solvent.
Crystal data and structure refinement details are reported in
Tables S1 and S2 (ESI†). The figures were generated using
4
5
Mercury CSD 3.3. All crystal data were deposited at Cam-
bridge Crystallographic Data Centre with assigned number
highly soluble in all common organic solvents, PAA is only soluble CCDC 2073781 (1-I), 2073782 (1-II), 2073784 (1-AA), 2073785
in water, alcohols and formamides. Selective extraction of azo- (1-AA-RT), 2073786 (1-MA), 2073783 (2).†
benzene units with dichloromethane produces a rapid replace-
Synthesis of 1 and 2
ment of the AP type chain–chromophore interactions with the
crosslinking AA type dimerization of the freed carboxylic groups, The tetrafluoroborate diazonium salts of p-toluidine (td) and
which contribute to the preservation of the SRG dimensional p-anisidine (ad) were synthesized and isolated according to a
stability. The same procedure applied to samples with higher azo reported procedure.4
6
content leads to unsatisfactory results, with severe loss of pattern
1. 2-Amino-4,6-dimethoxypyrimidine (1.55 g, 10 mmol) was
quality and regularity due to the excessive volume fraction of the dissolved in 1,2-dichloroethane (70 ml). td (2.93 g, 1.0 equiv.)
material occupied by the azobenzene.
was added in small portions and reaction stirred overnight at
room temperature. The hydrotetrafluoroborate product was
deprotonated and purified by recrystallization in ethanol/water
Experimental
in the presence of excess triethylamine, giving large plate-like
1
crystals. Yield: 77%. H NMR (DMSO-d
6
) d 7.52 (d, 2H); 7.32
All the reagents were commercially available and used without
further purification: p-toluidine, 2-amino-4,6-dimethoxypyri-
midine, tetrafluoroboric acid 48% aq. sol. (Alfa Aesar), p-ani-
13
(s, 2H); 7.27 (d, 2H); 3.91 (s, 6H); 2.35 (s, 3H). C NMR
DMSO-d ) d 164.3, 161.2, 151.9, 138.7, 129.5, 121.3, 112.9,
(
6
5
3.9, 20.8.
. The same procedure for the synthesis of 1 was applied
using an equimolar amount of ad to give 2 in 71% yield.
sidine, polyacrylic acid average M
w
1800 (Merck). Optical
2
observations were performed by using a Zeiss Axioscop polariz-
ing microscope equipped with a FP90 Mettler heating stage.
1
6
H NMR (DMSO-d ) d 7.61 (d, 2H); 7.25 (s, 2H); 7.02 (d, 2H);
1
13
H and C NMR spectra were recorded on Varian Inova 500
500 MHz) and Bruker Avance III TM HD (400 MHz) spectro-
meters. UV-Visible spectra were recorded with a JASCO V-750
13
6
3.91 (s, 6H); 3.81 (s, 3H). C NMR (DMSO-d ) d 164.2, 161.0,
(
160.2, 148.0, 122.9, 114.2, 112.8, 55.4, 53.8.
0
spectrometer. The FT-IR measurements were performed on a Synthesis of 3-(4 -hydroxyphenylazo)pyridine
Thermo Nicolet 5700 FT-IR spectrometer. Solid samples were
To a solution of 3-aminopyridine (2.51 g, 26.7 mmol) in H O
2
dispersed in KBr tablets, polymeric materials were cast from
solutions. Thin films of azopolymers were prepared by spin
coating DMF solutions of the polymers on glass substrates,
using a Laurell WS-650Mz-23NPP spin coater.
(
11 ml), aqueous HBF
mixture was stirred at 5 1C. A solution of sodium nitrite (1.84 g,
6.7 mmol) in 5 ml water was slowly added, the mixture was
4
50 wt% was added (9.1 ml) and the
2
stirred at 5 1C for 45 min and then added to a solution of
phenol (2.51 g, 26.7 mmol) and NaOH (3.74 g, 93.5 mmol) in
X-Ray diffraction analysis
1
50 ml H O. After 30 min stirring, the pH was lowered to 7 with
2
Single crystals of 1-I and 1-II suitable for X-ray diffraction HCl. The orange precipitate was filtered and washed with water.
1
analysis were obtained by slow cooling of a heptane solution Yield: 66%. H NMR (DMSO-d ) d 10.49 (s, 1H); 9.02 (s, 1H);
6
(
or evaporation from acetone) and by recrystallization from 8.66 (d, 1H); 8.08 (d, 1H); 7.84 (d, 2H); 7.55 (m, 1H); 6.96 (d, 2H).
1
3
ethanol/water, respectively. Single crystals of 2 were obtained
6
C NMR (DMSO-d ) d 161.6, 151.1, 147.5, 145.9, 145.4, 126.6,
by slow evaporation from acetone, while those of the complexes 125.2, 124.4, 116.1.
J. Mater. Chem. C
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Journal of Materials Chemistry C
Paper
Synthesis of 3
migration and SRG inscription efficiency compared to a single
H-bonded system based on the same polymer matrix and
positioning of the azomolecule relative to the chain. Rapid
and straightforward removal of the light absorbing chromo-
phores from a photopatterned sample was enabled by the
orthogonal solubility properties of the polyacrylic acid and
the dye component and demonstrated through simple rinsing
of the thin film samples with dichloromethane.
0
3
-(4 -Hydroxyphenylazo)pyridine (1.20 g, 6.00 mmol) and K
2
CO
3
(
(
1.66 g, 12.0 mmol) were added to 30 ml DMF. Dimethyl sulfate
0.757 g, 6.00 mmol) were slowly added and the mixture was
stirred at room temperature for 48 h. After addition of water
50 ml) the mixture was extracted with ethyl acetate. The
(
organic phase was washed three times with water, anhydrified
with Na SO and the solvent removed in vacuo. Ethanol (10 ml)
2
4
was added to the liquid residue and a solid precipitated by
addition of a solution of sodium acetate (1 g) in 30 ml H O. The
brown solid was treated with 600 ml boiling heptane. After
2
Conflicts of interest
cooling, the residue was filtered and the volume was reduced to There are no conflicts to declare.
0 ml. By cooling the solution, the product was crystallized as
2
1
6
an orange solid. Yield: 62%. H NMR (DMSO-d ) d 9.07 (m, 1H);
Acknowledgements
8
7
1
.71 (m, 1H); 8.16–8.13 (m, 1H); 7.94 (d, 2H); 7.62–7.59 (m, 1H);
1
3
.17 (d, 2H); 3.89 (s, 3H). C NMR (DMSO-d ) d 162.5, 151.4,
6
This work has been financially supported by the European
Research Council (ERC) under the European Union’s Horizon
47.4, 146.2, 146.0, 126.7, 124.9, 124.9, 124.51, 114.7, 55.7.
2
020 research and innovation programme ‘‘METAmorphoses’’,
SRG inscription
grant agreement no. 817794. This work has been supported by
Fondazione Cariplo, grant no 2019-3923.
Thin films for SRG inscription experiments were prepared by
spin coating filtered 25 wt% DMF solutions of the polymers on
glass slides (film thickness 880–950 nm). The samples were
kept under vacuum at room temperature for 24 h to remove
solvent traces. The interference pattern was set up with two
p polarized laser beams of 5.5 mW power and 2 mm section
each (irradiance 175 mW cm , wavelength 457 nm, from
Samba source of Cobolt 05-01 Series by H U¨ BNER PHOTONICS)
hitting the sample at an incidence angle of 13140 . The first
order diffraction efficiency was monitored with a He–Ne laser
beam impinging normally on the growing grating and the
diffracted light power was monitored by means of biased
photodetectors (DET36A2 model from Thorlabs, based on fast
PIN Si photodiodes) placed at the angular directions of the zero
and the two first diffraction orders for the 633 nm radiation.
The DE was calculated as the power ratio of the He–Ne
diffracted beam and the transmitted beam prior to exposure
to the 457 nm intensity pattern, by averaging the signal of both
Notes and references
1
2
3
4
5
6
7
A. Natansohn and P. Rochon, Chem. Rev., 2002, 102,
139–4176.
4
ꢁ
2
M. O’Neill and S. M. Kelly, J. Phys. D: Appl. Phys., 2000, 33,
R67–R84.
L. Corvazier and Y. Zhao, Macromolecules, 1999, 32,
0
3195–3200.
T. H. Ware, M. E. McConney, J. J. Wie, V. P. Tondiglia and
T. J. White, Science, 2015, 347, 982–984.
H. Zeng, O. M. Wani, P. Wasylczyk, R. Kaczmarek and
A. Priimagi, Adv. Mater., 2017, 29, 1701814.
X. Meng, A. Natansohn and P. Rochon, Supramol. Sci., 1996,
3, 207–213.
M. Eich, J. Wendorff, B. Reck and H. Ringsdorf, Makromol.
Chem., Rapid Commun., 1987, 8, 59–63.
A. Shishido, Polym. J., 2010, 42, 525–533.
ꢁ
1 and +1 first orders.
8
9
S. Hvilsted, C. S ´a nchez and R. Alcal ´a , J. Mater. Chem., 2009,
1
9, 6641–6648.
Conclusions
1
0 P. Rochon, E. Batalla and A. Natansohn, Appl. Phys. Lett.,
Here we demonstrated the possibility to exploit polyacrylic acid
1995, 66, 136–138.
as an affordable polymer matrix to produce supramolecular 11 S. L. Oscurato, M. Salvatore, P. Maddalena and A. Ambrosio,
azopolymers with high performances and versatility, through a Nanophotonics, 2018, 7, 1387–1422.
molecular recognition approach based on a hydrogen bond 12 N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu,
supramolecular synthon. These materials can be prepared with
the desired chromophore content without undergoing phase
L. Li, L. Samuelson, J. Kumar and S. K. Tripathy, J. Mater.
Chem., 1999, 9, 1941–1955.
separation or chromophore aggregation until equimolar ratio. 13 L. M. Goldenberg, V. Lisinetskii, Y. Gritsai, J. Stumpe and
The proposed system exploits for the first time a double weak S. Schrader, Adv. Mater., 2012, 24, 3339–3343.
interaction acting on two positions of the aromatic ring of the 14 L. M. Goldenberg, V. Lisinetskii, Y. Gritsai, J. Stumpe and
azobenzene unit. This approach promotes a highly directional S. Schrader, Opt. Mater. Express, 2012, 2, 11.
chromophore–chain interaction, while also preventing any 15 S.-I. Na, S.-S. Kim, J. Jo, S.-H. Oh, J. Kim and D.-Y. Kim,
degree of freedom with respect to rotation around the mole- Adv. Funct. Mater., 2008, 18, 3956–3963.
cular long axis of azobenzene units. This stronger and more 16 S. L. Oscurato, M. Salvatore, F. Borbone, P. Maddalena and
rigid constraint resulted in a huge improvement in mass
A. Ambrosio, Sci. Rep., 2019, 9, 6775.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
ˇ
´
1
1
1
2
2
2
2
7 M. Salvatore, F. Borbone and S. L. Oscurato, Adv. Mater. 32 L. Cechova, J. Kind, M. Dracınsk ´y , J. Filo, Z. Janeba,
ˇ´
Interfaces, 2020, 7, 1902118.
8 S. Lee, H. S. Kang and J.-K. Park, Adv. Mater., 2012, 24,
C. M. Thiele, M. Cig ´a nˇ and E. Proch ´a zkov ´a , J. Org. Chem.,
2018, 83, 5986–5998.
33 D. R. C. Matazo, R. A. Ando, A. C. Borin and P. S. Santos,
J. Phys. Chem. A, 2008, 112, 4437–4443.
34 N. J. Dunn, W. H. Humphries, A. R. Offenbacher, T. L. King
and J. A. Gray, J. Phys. Chem. A, 2009, 113, 13144–13151.
35 N. Sundaraganesan, K. Sathesh Kumar, C. Meganathan and
B. Dominic Joshua, Spectrochim. Acta, Part A, 2006, 65,
1186–1196.
2069–2103.
9 S. L. Oscurato, F. Borbone, P. Maddalena and A. Ambrosio,
ACS Appl. Mater. Interfaces, 2017, 9, 30133–30142.
0 Q. Zhang, X. Wang, C. J. Barrett and C. G. Bazuin,
Chem. Mater., 2009, 21, 3216–3227.
1 J. Gao, Y. He, F. Liu, X. Zhang, Z. Wang and X. Wang,
Chem. Mater., 2007, 19, 3877–3881.
2 O. Kulikovska, L. M. Goldenberg and J. Stumpe, 36 Q. Huo, L. Dziri, B. Desbat, K. C. Russell and R. M. Leblanc,
Chem. Mater., 2007, 19, 3343–3348. J. Phys. Chem. B, 1999, 103, 2929–2934.
3 A. Priimagi, G. Cavallo, A. Forni, M. Gorynsztejn-Leben, 37 S. Mishra, J. L. Kuo and G. N. Patwari, Phys. Chem. Chem.
M. Kaivola, P. Metrangolo, R. Milani, A. Shishido, Phys., 2018, 20, 21557–21566.
T. Pilati, G. Resnati and G. Terraneo, Adv. Funct. Mater., 38 C. Greve, E. T. J. Nibbering and H. Fidder, J. Phys. Chem. B,
012, 22, 2572–2579. 2013, 117, 15843–15855.
4 A. Priimagi, K. Lindfors, M. Kaivola and P. Rochon, 39 E. R. Berg, S. A. Freeman, D. D. Green and D. J. Ulness,
ACS Appl. Mater. Interfaces, 2009, 1, 1183–1189. J. Phys. Chem. A, 2006, 110, 13434–13446.
5 J. Vapaavuori, V. Valtavirta, T. Alasaarela, J.-I. Mamiya, 40 C. Barrett, A. Natansohn and P. Rochon, Chem. Mater., 1995,
2
2
2
A. Priimagi, A. Shishido and M. Kaivola, J. Mater. Chem.,
011, 21, 15437.
6 J. Gao, Y. He, H. Xu, B. Song, X. Zhang, Z. Wang and
X. Wang, Chem. Mater., 2007, 19, 14–17.
7 A. Priimagi, S. Cattaneo, R. H. A. Ras, S. Valkama, O. Ikkala
and M. Kauranen, Chem. Mater., 2005, 17, 5798–5802.
8 J. E. Stumpel, M. Saccone, V. DIchiarante, O. Lehtonen,
7, 899–903.
2
41 N. Zettsu, T. Ogasawara, N. Mizoshita, S. Nagano and
T. Seki, Adv. Mater., 2008, 20, 516–521.
42 A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano,
C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni,
G. Polidori and R. Spagna, J. Appl. Crystallogr., 1999, 32,
115–119.
2
2
2
M. Virkki, P. Metrangolo and A. Priimagi, Molecules, 2017, 43 G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem.,
2, 1844. 2015, 71, 3–8.
9 M. Saccone, V. Dichiarante, A. Forni, A. Goulet-Hanssens, 44 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard
G. Cavallo, J. Vapaavuori, G. Terraneo, C. J. Barrett, and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
G. Resnati, P. Metrangolo and A. Priimagi, J. Mater. Chem. 45 C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington,
2
2
C, 2015, 3, 759–768.
0 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34,
P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J. van
de Streek and P. A. Wood, J. Appl. Crystallogr., 2008, 41,
466–470.
3
2311–2327.
31 N. K. Duggirala, J. Li, N. S. Krishna Kumar, T. Gopinath and 46 W. Erb, A. Hellal, M. Albini, J. Rouden and J. Blanchet,
R. Suryanarayanan, Chem. Commun., 2019, 55, 5551–5554. Chem. – Eur. J., 2014, 20, 6608–6612.
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